The grass are the cheap seats for a reason, and its not cause thats where the atmospheer is.
Well alot of people that don't know the stadium, will buy pavilion seats as they know they'll at least have a seat. Then they pitch up and see hey, look it's not so full, we can sit right next to the cheer-leader girls and even get some signatures from the players standing right in front of me....and then they end up on the grass. The pavilion is a little "removed" from the action at times.
The grass isn't that bad, just bring your own chairs and it's better than the pavilion by far IF it's not overly crowded.
So i won't be surprised if people pay the few $ extra to ensure that if the game is "sold out completely" and totally crowded, they have their seats and at the same time if the game is not crowded they can go chill on the grass...
Either way, all the IPL matches i watched, the stadium was supposedly filled to capacity. The amusing part, the pavilion was EMPTY, even during the semi finals!